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If it's not a printing error and the cancel is to be blame here,
I do not think the cancel is any factor here. At best, it might hide that the unprinted area is not an extension of the 'h' but just a small unprinted spot above and slightly towards the left of it.
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then why is the dot positioned directly on the top of the letter ,not at some other part of the text ?
Dumb coincidence. It does look like either some foreign matter got between the stamp and the printing plate. We are looking for a small piece of something that has been printed in a purple colour.
That is assuming it is not damage to the stamp surface: the white area of the spot seems less bright than the 'h' which might be caused by damaged surface coating. In that case it is not a minor flaw, but a damaged stamp. There are some small white spots to the lower left of the printed area as well. If you have a good microscope, you might want to check whether this is an unprinted area or a damaged stamp surface.